Energy-absorbing laminates for use as safety glass is well known. Customarily, these products are formed from transparent materials of different compositions and are composed of an outer layer of one or more plies and an inner layer which is bonded together with the outer layer by a resinous interlayer. Usually, a relatively thick outer layer is combined with at least one relatively thin inner layer.
For bulletproof laminated safety glass and for windshields, it is generally believed that the inner layer should be a resinous material such as a polycarbonate in order to avoid spalling. Furthermore, it was believed that if the inner layer is glass, an antispall covering of a resinous material is necessary to reduce the risk of flying glass splinters upon impact.
The use of so-called safety glazing or penetration resistant glazing for windows, windshields, and the like using multiple layers of polycarbonate, glass and other resinous materials is well known. For example, glass-polycarbonate resin laminates are described in U.S. Pat. Nos. 4,663,228 and 3,666,614.
In U.S. Pat. No. 3,520,768 there are described laminates of relatively thick glass having a comparatively thin polycarbonate foil as the cohering layer. While generally useful, these laminates suffer from an inability to withstand multiple shots, especially when struck by high velocity bullets such as those fired from rifles. Thus, for example, in prior art laminates utilizing thick forward-facing (impact receiving) glass plies, multiple shots at the thick glass front ply cause much glass cracking and removal of the glass from the laminated structure, making it vulnerable to repeat hits. In order to withstand the repeated hits, the laminates had to be extremely thick and heavy. Even these thick and heavy laminates were not entirely successful, since spalling on the back side or downstream face of said laminates occurred with the resultant danger of injury due to this spalling to persons behind these laminates.
Thus, there is a need for laminates which are capable of withstanding repeated high velocity strikes without penetration and/or spalling of the back or downstream layer.
U.S. Pat. No. 4,125,669 to Triebel et al discloses a laminated safety glass which utilizes a thick outside layer of silicate glass which is bonded to a polycarbonate pane of at least 1.5 mm thick.
U.S. Pat. No. 4,312,903 to Molari discloses impact resistant, double glazed structures comprising a plurality of laminae selected from polycarbonate, glass and solid resinous materials. The glass laminae faces the direction of impact and has a thickness from about 30 to 220 mils. The structure utilizes relatively thick outward layers and relatively thin inward layers.
U.S. Pat. Nos. 4,663,228 and 4,799,346 to Bolton et al, which are herein incorporated by reference, disclose laminated safety glass structures which are preferable for use in the present invention. However, the inward layers disclosed in this patent have a thickness which would not provide the anti-spalling characteristics of the invention.